1. Field of the Invention
This invention relates to an optical semiconductor device having a buried quantum well structure, and more particularly to an optical semiconductor device designed for improving the characteristic of a laser of a multiple quantum well structure or the like.
2. Description of the Related Art
In recent years, a so-called quantum well semiconductor laser having at least one quantum well structure obtained by forming a well layer of a film thickness less than the de Broglie wavelength of electrons in an active layer and barrier layers whose forbidden band width is larger than that of the well layer and which are disposed on both sides of the well layer has been developed. The quantum well semiconductor laser has various advantages over a semiconductor laser of a double hetero structure having no quantum well formed in the active layer in that the oscillation threshold value can be lowered, the modulation bandwidth can be widened, the oscillation spectrum width can be narrowed, the temperature characteristic can be improved, and a high output power can be obtained, for example.
Further, recently, the degree of freedom of the oscillated wavelength may be enhanced and the characteristic may be further improved by using material which does not lattice-match with the substrate to form the quantum well layer. In general, if a material which does not lattice-match with the substrate exists near the active layer, a large number of lattice defects occur and the reliability and characteristic of the laser may be significantly deteriorated. However, even when a material which does not lattice-match with the substrate is used in the quantum well layer, the layer may be elastically strained so as not to generate lattice defects which may deteriorate the reliability and characteristic of the laser if the thickness of the layer is smaller than a critical thickness The result of calculations obtained by use of a model of Matthews and Blakeslee may be used as one measure of the critical thickness
A high power laser and a laser with an extremely small threshold value are realized by use of an InGaAs/AlGaAs semiconductor laser formed on a GaAs substrate of 0.98 .mu.m bandwidth utilizing the above-described strained quantum well structure. Various possibilities of changing the TE/TM mode characteristic and changing the switching characteristic and absorption characteristic can be obtained by introducing a strained optical waveguide layer in an optical semiconductor device other than the semiconductor laser, for example, a semiconductor laser amplifier, optical switch and optical modulator
In the case of an InGaAs/InGaAsp series semiconductor laser formed on the InP substrate mainly used in the optical fiber communication, a so-called buried heterostructure in which the right and left portions of the active layer are filled with material having a larger forbidden bandwidth than the active layer is frequently used in order to obtain the single lateral mode oscillation of small threshold value. In this case, the strained quantum well layer comes to have a strained hetero interface on the side surface thereof and a large strain and stress are generated.
Now, a laser of In.sub.0.7 Ga.sub.0.3 As strained quantum well structure formed on a (001) InP substrate 1 shown in FIGS. 23A and 23B is explained in detail as an example. The semiconductor laser is constructed by forming a multi-layered structure of an active layer (optical waveguide layer) 4 of strained quantum well structure, p-type InP clad layer 5 of 1.5 .mu.m thickness and p-type InGaAsP (composition corresponding to a photoluminescence wavelength of 1.2 .mu.m (1.2 .mu.m-wavelength composition)) contact layer 6 of 0.8 .mu.m thickness on the n-type InP substrate 1 (which is also used as a clad layer) and burying the laminated structure in an Fe-doped semi-insulative InP layer 7. The active layer 4 has undoped In.sub.0.7 Ga.sub.O.3 As well layers 2 of 4.2 nm thickness and undoped InGaAsP (1.2 .mu.m-wavelength composition) barrier layers 3 which lattice-match with InP and are respectively disposed on both sides of a corresponding one of the well layers.
The barrier layers 3 are formed with a thickness of 12 nm between the well layers 2 and a thickness of 20 nm on each of the uppermost and lowermost well layers and thus have a total thickness of 76 nm. Therefore, the total thickness of the active layer 4 is 92.8 nm. Further, the width of the buried active layer 4 is 2 .mu.m. Electrodes 11 and 12 for current supply are formed on opposite sides of the substrate. Further, a laser resonator (optical cavity) with a length of 1 mm is formed by cleavage.
The lattice constant of the InP substrate is 0.58688 nm and the lattice constant of In.sub.0.7 Ga.sub.0.3 As is 0.59381 nm. Assuming now that there is an infinite plane having no side surface, the well layer 2 is elastically strained since it is as thin as 4.2 nm. The degree of strain is .epsilon..sub.xx =.epsilon..sub.yy =-0.01167, .epsilon..sub.zz =-2(C.sub.12 /C.sub.11)x.epsilon..sub.xx =0.011974, .epsilon..sub.yz =.epsilon..sub.zx =.epsilon..sub.xy =0. C.sub.12 /C.sub.11 =0.504 is the Poison's ratio of In.sub.0.7 Ga.sub.0.3 As. As a result, the lattice constant of the strained In.sub.0.7 Ga.sub.0.3 As well layer 2 becomes equal to that of InP in the xy plane and the lattice constant in the z direction is 0.60092 nm.
However, in practice, the active layer 4 is buried in the Fe-doped InP blocking layer 7 in a stripe form with a width of 2 .mu.m. As a result, lattice-mismatching occurs in the boundary between the well layer 2 and the blocking layer 7. Since the thickness of the well layer 2 is 4.2 nm, it may be formed of approx. 6.99 unit layers on average. In the face-centered cubic zinc blende structure, four atomic layers are present in the lattice interval a with a positional deviation in the &lt;001&gt; direction, and in this case, four layers (lattice constant a) which are aligned with each other are counted as one unit lattice layer.
When the thickness is divided by the lattice constant of InP, 7.16 unit lattice layers can be obtained. The well layer 2 has four layers and therefore lattice-mismatching corresponding to the 0.68 unit lattice layer will occur in the side surface of the active layer 4. Since the width of the active layer 4 is as large as 2 .mu.m, the entire portion of the active layer 4 will not be elastically compressed. As a result, lattice defects such as dislocations may tend to occur near the side surface of the active layer 4.
Since the degree of lattice-mismatching of the upper side surface of the strained quantum well layer is larger than the 0.5 unit lattice layer, the lattice plane of the strained quantum well layer becomes nearer to a lattice plane of the blocking layer lying directly above the strained quantum well layer and will be more easily connected to the lattice plane on the side surface thereof, and the possibility of occurrence of dislocations becomes high. With such lattice defects, segregation of impurity tends to occur. Further, with such lattice defects, significant reduction in the reliability and deterioration in the laser characteristic such as reduction in the light emission efficiency rise in the oscillation threshold value and reduction in the differential gain may occur.
In an optical semiconductor device other than an semiconductor laser, defects due to the lattice-mismatching occur in the side surface of the buried strained quantum well optical waveguide layer to cause various demerits. For example, in a buried strained quantum well optical waveguide, an increase in the absorption coefficient and an increase in the light scattering coefficient may be caused by the defects. Further, in a light detection device having a buried strained quantum well light absorption layer, various problems such as an increase in a generation recombination dark current caused by the defects on the side surface, irregularity of the internal electric field caused by impurity segregated by the defects, reduction in the quantum efficiency due to recombination in the defects and reduction in the reliability may occur.
Further, in a semiconductor device other than the optical semiconductor device, a strained semiconductor layer such as a pseudomorphic HEMT strained channel layer or a strained In(Ga)As ohmic contact layer is used to improve the characteristic. When the strained semiconductor layer is buried in part of the substrate instead of the entire portion of the substrate, various problems such as reduction in the carrier lifetime, reduction in the electron mobility due to an increase in the scattering centers, an increase in noises, and reduction in the reliability which are caused by segregation of impurity and lattice defects due to the lattice-mismatching on the side surface may occur.
In a quantum well laser having InGaAsP or InGaAs at the well layer widely used in the optical communication field, the well barrier of the conduction band is low and the well barrier of the valence band is high so that overflow of electrons to the barrier layer and optical waveguide layer may easily occur and the injection efficiency of holes into the well layer will become low. For this reason, the utilization efficiency of carriers becomes low and the effect of reduction in the oscillation threshold value and extension of the modulation bandwidth is not significant in comparison with a case of a bulk type double hetero semiconductor laser having the same oscillation wavelength.
Since the injection efficiency of holes into the well layer becomes low when stimulated emission becomes large and the number of carriers becomes insufficient, a difference in the hole density occurs between the wells and the gain saturation .epsilon. for light becomes large. This means that damping becomes large and the frequency bandwidth will not be extended irrespective of an increase in the differential gain caused by the effect of the quantum well. A high power laser may have a preferable characteristic when it is of long resonator and a large number of wells are used, but since the above problems are present, the number of wells cannot be fully increased in practice.
In general, since saturation of the gain G with respect to the carriers is significant in the quantum well laser, the differential gain becomes significantly deteriorated when the oscillation threshold carrier density becomes high. Therefore, reduction in the oscillation threshold value is important in view of the high-speed response characteristic. Since the oscillating condition can be expressed by .GAMMA.G=.alpha., the optical total loss c must be set to a small value to decrease the gain necessary for the oscillation in order to reduce the oscillation threshold value.
In a quantum well laser, particularly, in a strained quantum well laser having strains in the well layer, the waveguide loss of the active layer can be suppressed to an extremely small value, but the loss may become large when the acceptor density of a p-clad layer is high. Since the optical confinement coefficient .GAMMA. of the quantum well laser is small, influence by absorption in the clad layer is extremely large. Therefore, in order to obtain a quantum well laser of low threshold value, it is necessary to reduce the acceptor density of the p-clad layer.
In this case, however, since a large band barrier is created in the hetero junction portion between the p-clad layer and the active layer, there occurs a problem that the injection efficiency of holes into the quantum well active layer is lowered. This means that it is difficult to form a quantum well laser which has a small threshold value and a high-speed response. In order to solve the problem of the barrier of the hetero interface, use of GRIN (graded index) is proposed, but in this case, precise control for the crystal growth becomes necessary and the process becomes complicated.
In addition, a problem that defects of dislocations tend to occur in the side surface of the active layer by the lattice-mismatching has occurred in the strained quantum well laser of buried structure. In particular, when a layer doped with a transition metal such as Fe is formed on the side surface, there occurs a possibility that composite defects of dislocations and the transition metal serving as an impurity deteriorate the laser characteristic.
Thus, in the conventional semiconductor device having the strained semiconductor layer of buried structure, defects such as lattice defects and impurity segregation caused by the lattice-mismatching of the side surface of the strained semiconductor layer may occur, thereby significantly deteriorating the characteristic and reliability.
Further, in the multiple quantum well laser of InGaAs or InGaAsP, the injection efficiency of holes into the well layer is low and a difference in the hole density tends to occur between the wells, thereby making it difficult to increase the number of wells. Further, it is difficult to realize a laser which has a small absorption loss and high injection efficiency of holes and reduction in the oscillation threshold value and the improvement of the high-speed response characteristic cannot be easily attained.